This invention relates generally to fiber optic grating systems and more particularly, to the measurement of environmental effects using fiber optic grating sensors. Typical fiber optic grating sensor systems are described in detail in U.S. Pat. Nos. 5,380,995, 5,402,231, 5,592,965, 5,841,131 and 6,144,026.
The need for low cost, a high performance fiber optic grating environmental sensor system that is capable of long term environmental monitoring, virtually immune to electromagnetic interference and passive is critical for such applications as moisture sensing and monitoring of adhesive bonds: Another advantage of these system is that when they are appropriately configured the frequency response of the system can be very high.
The present invention includes multi-axis fiber grating sensors that may be used to sense axial strain and temperature, or axial and transverse strain simultaneously to detect chemical changes such as moisture by using appropriate transducers or changes to the structural integrity of coatings such as adhesive bonds. Means are also described to multiplex these fiber grating sensors allowing high sensitivity and high speed measurements to be made.
In U.S. Pat. Nos. 5,380,995 and 5,397,891 fiber grating demodulation systems are described that involve single element fiber gratings and using spectral filters to demodulate fiber gratings. The present invention includes means to extend the demodulation system to multiple fiber grating sensors operating at high speed on a single fiber line. In U.S. Pat. Nos. 5,591,965, 5,627,927 the usage of fiber gratings to detect more than one dimension of strain is described. These ideas are extended in U.S. Pat. Nos. 5,869,835, 5,828,059 and 5,841,131 to include fibers with different geometries that can be used to enhance sensitivity or simplify alignment procedures for enhanced sensitivity of multi-parameter fiber sensing. In U.S. patent application Ser. No. 09/176,515, xe2x80x9cHigh Speed Demodulation Systems for Fiber Grating Sensorsxe2x80x9d, by Eric Udd and Andreas Weisshaar means are described to process the output from multi-axis fiber grating sensors for improved sensitivity. All of these patents teaching are background for the present invention which optimizes the fiber grating sensor for optimum response to strain changes induced by changes in the state of its coating or surrounding media to form water/chemical sensors and monitor the status of adhesive joints through measurements of strain interior to the bond.
The present invention consists of an optical fiber whose axial, transverse and or temperature sensitivity has been optimized through the construction of the optical fiber or mechanical mechanisms to enhance sensitivity. High speed demodulation is provided by wavelength division multiplexing of these fiber grating sensors using series of fiber grating filters. The spectral filters are arranged so that each fiber grating sensor has a corresponding filter to match it, allowing higher speeds and sensitivity than many current approaches. To sense transverse strain at high speeds in birefringent optical fiber, the two spectral peaks associated with the fiber gratings are tracked individually by locking onto its preferred polarization state.
Therefore, it is an object of the present invention to monitor changes in moisture or chemical content of an environment through measured strain changes.
Another object of the invention is to provide a means of monitoring bond lines for degradation.
Another object of the invention is to provide means to measure changes in several fiber grating sensors at high speed and with high sensitivity simultaneously in a single fiber.
Another object of the invention is to measure transverse strain as well as axial strain at high speed and with high sensitivity.